The present invention relates to a voice and data transmission system which is used in local areas for communication to multiplexed voice and data sent from subscribers.
The local area includes terminal equipment, Data Circuit terminal Equipment (hereinafter abbreviated as DCE), a line, Office Channel Equipment (hereinafter abbreviated as OCE) and office equipment.
The terminal equipment is composed of telephone equipment and data equipment. The office equipment is mainly composed of an exchange and data gathering equipment.
DCE has the function of sending the digitalized voice sent from the telephone equipment to the line after it is combined with the data sent from the data terminal and separating the digital signal sent from the line into the voice and the data.
OCE has the function of separating the voice and data sent from the line on the office side and sending these signals to the exchange or data gathering equipment for data multiplexing, or to multiplex and send the voice sent from the exchange and data sent from gathering equipment and then send these signals to the line and supply the supervisory current to the DCE.
This supervisory current is returned to the OCE from the constant current source provided to the OCE through the lines. The OCE measures the supervisory current value and detects that a failure has occurred in the like or the DCE from the fact that such supervisory current value is lower or becomes OFF.
FIG. 1 illustrates a system connecting a telephone equipment 1 and an exchange 3.
FIG. 2 illustrates a system connecting a data equipment 4 and data gathering equipment 8.
FIG. 3 is a block diagram of subscriber transmission side of the prior art.
When the same subscriber installs the telephone equipment system and data equipment system, individual installation of the system as illustrated in FIG. 1 and FIG. 2 results in a rise in line cost.
Therefore, as illustrated in FIG. 3, the telephone equipment 1 has been connected with the DCE 5 together with the data equipment 4 by using in common the line of telephone equipment of the prior art, the DCE 5 with the line 2, the line 2 with the OCE 7 and the OCE 7 separately with the exchange 3 and the data gathering equipment 8.
In the DCE 5, the voice sent from the telephone equipment 1 is encoded and converted to a digital signal, it is then multiplexed together with the digital signal sent from the data equipment 4 and is transmitted to the line 2.
Meanwhile, in the OCE 7, the digital signal sent from the line 2 is separated into the voice sent from the telephone equipment 1 and the digital signal sent from the data equipment 4. The former is sent to the exchange 3, while the latter to the data gathering equipment 8.
It should be noted in FIG. 2 and FIG. 3 that both the DCE and the OCE are provided in both figures but the DCE and the OCE in FIG. 2 are different in the functions from those in FIG. 3.
Namely, the DCE 6 in FIG. 2 executes code conversion for sending the digital signal sent from the data equipment 4 to the telephone line 2, while the OCE 9 executes reverse code conversion and sends the received data to the data gathering equipment 8.
On the other hand, the DCE 5 in FIG. 3 also has the function, in addition to the function described previously, of converting the voice sent from the telephone equipment 1 into the digital signal and multiplex it with the digital data from the data equipment 4. Moreover, the OCE 7 separates the multiplexed voice and data and then sends these signals to the exchange 3 or data gathering equipment 8.
However, the voice and data transmission system illustrated in FIG. 3 connects both telephone equipment 1 and data equipment 4 to the DCE 5 and therefore cannot realize communication by telephone and sends data when operation of the DCE 5 stops. Operation of the DCE 5 stops when the commercial power supply to the DCE 5 stops due to the power failure or when the user of data equipment 4 terminates the use of the equipment and turns OFF the power switch of the DCE 5.
The DCE 5 is essential equipment for transmitting the data from data equipment 4 to the line 2. Meanwhile the DCE 5 is not always essential equipment for the telephone equipment 1 as will be apparent from the structure of FIG. 1.
Therefore, it is desired that a system, which assures operation of at least the telephone equipment 1 even when operation of DCE 5 stops, be developed.